Only guard is on the axle sheave , without it you would be throwing belts, I suspect weak return spring (or missing?) On the mule drive, pics would help here's a couple pics of the mule drive with return spring shown
Sounds like it could be wrong belts or the return spring on the idler is worn or missing as stated by wrenchinonit and one last thing could be the pto pulley guard is bent a little and pushing the belt up into the pulley enough to allow some spin. Also there is a couple of guides for the belts see if they have been bent and putting tension on the belt when disengaged.
The finger that pushes the belt when dis-engaged is either broken off or worn off (1st pic just to the left of center, second pic just below center). It should extend slightly past the width of the 3 groove sheave. It's supposed to be adjusted so the finger touches the belts when dis-engaged to stop the rotation.
Those pics are of a broken bolster unit my intentions were to show you the return spring, the unit is junk. Correct on belt guides I believe their purpose is to help keep the belts in place is all ,you said in your previous post the tractor had set I don't know how you went after the belt change but it's also possible the mule drive axis points are hanging up, rusty, shoot all the pivot points with PB or another good rust displacer , make sure the spring ( shown) is allowing disengagement. OB is right there is a small clevis on a threaded rod attached to the backside of the pto lever that will allow tension to be added or taken off the large coil spring shown in one pic, soak that spring good as well.